Since the mid 1990's there has been an increase in the use of explosives by criminal organisations against civilian and military targets throughout the World. Their use results in death, injury and destruction of property and buildings. Previously, mitigation of explosion relied upon intelligence and police detection to provide warning of impending attack but recent events make it clear that intelligence and police operations alone cannot be relied upon to prevent explosions. Moreover, some explosions are caused simply by accident, e.g. gas or chemical explosions, and it would be useful if the consequences of such accidental explosions could also be minimised.
Conventional construction can give rise to buildings which will withstand many types of impact but it is still difficult to minimise the effects of explosions. Of particular importance and concern are the windows especially in high rise buildings. Windows are a major cause of trauma and injury caused by explosions; the fragmentation of pieces of glass not only causes death but many other permanent injuries such as loss of eyesight, organ trauma etc.
It is well known therefore for buildings and in particular windows to be protected against explosion damage by materials which mitigate their effects.
One option for minimising the problem of glass fragmentation utilises an adhesive film made of a polyester composite material which can be applied to the inside of a window to contain glass fragments. Such films do not however prevent injury caused by fragments of masonry from cladding or from fragments falling from a height.
Certain other elastomeric polymer materials have been suggested for use as building cladding to prevent damage caused by explosions. The elastomer material is a highly ductile polymer that can be sprayed onto building surfaces including windows to prevent injury caused both by flying glass and masonry. The polymer employed is based on a polyurea and may be suitable for use with temporary structures as well as concrete buildings (Polymer materials for structural retrofit, Knox et al, Air Force Research Laboratory). The polymer is not transparent however and its use on windows is not desirable. Moreover, experiments using the polymer have not shown a reduction in pressure effects inside a building.
There are a number of reports of conventional fire fighting foams being employed as pressure mitigants (Journal of Explosives Engineering, Vol 26, No. 3, 1999). Such foams have the additional advantage of preventing fires often associated with explosions. However, the use of these foams requires that the explosive can be surrounded by the foam in a contained environment. Whilst this is possible when the source of an explosion is identified, where an explosion occurs without warning these foams cannot be used. Nor do these foams allow access to an explosive source by persons working to mitigate an accident or defuse a device controlled by criminals.
A somewhat similar system is sold under the trade name Hydrosuppressor. The system involves spraying the explosive or spraying the area in the vicinity of the explosive with water from various angles. Again however, this technique relies on the identification of an existence of a threat of an explosion prior to any explosion taking place.
A more conventional pressure protection system involves coating windows with a woven fabric mesh which acts to catch fragments of glass during any explosion. However, the mesh necessarily obscures the view through the window since it is not transparent. Moreover, the material does not cause any reduction in primary pressure within a building and hence offers no protection against direct pressure effects.
Recently, pressure impulse mitigation has been significantly improved by the use of blast net curtains and by the retrofitting of laminated glass. However, whilst net curtains provide some protection against fragmentation from glass they do not protect building integrity. Also, laminated glass cannot be used higher than about 7 storeys since it falls in total window size, i.e. does not fragment. This is potentially lethal to those in the street below.
There remains a need therefore for novel classes of pressure mitigation materials to be designed, which overcome the limitations of any of the present generation of such materials including but not limited to those described herein, and in particular to provide protection against zero warning explosions. Moreover, with the increase in criminal activity, the use of pressure impulse mitigation materials in construction may become common place and hence there remains a need to devise cheap, non-toxic materials for pressure impulse mitigation.